Rigidity Index of Soft Remolded Clays during Thixotropic Hardening-Paper

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The study investigated how the rigidity index of fully remolded soft clays changes as they gain strength through thixotropic hardening. Six natural and manufactured clays with a range of plasticities were cured at constant water content and temperature. Undrained shear strength was measured periodically for up to 64 days using fall-cone tests, while bender elements provided repeated, nondestructive measurements of shear-wave velocity. These measurements were used to calculate the small-strain shear modulus and rigidity index, defined as \(I_R = G_{\max}/s_u\).<br /><br />All clays gained strength and stiffness over time, with the most rapid increases occurring during the first two to four days. At about 32 days, strength had increased to 2.0–3.5 times its initial value, and shear-wave velocity to 1.4–2.3 times its initial value. The time-dependent changes in both properties were well described by a five-parameter Burgers-type model. In general, the relative gains in strength and velocity were smaller for clays with higher liquid limits.<br /><br />Despite increasing strength and stiffness, each clay’s rigidity index remained approximately constant throughout curing. Rigidity index values generally decreased as plasticity increased. Adjusting the measured small-strain moduli to estimate modulus at larger strain produced rigidity-index values broadly consistent with an empirical relationship for intact clays. This suggests that intact and thixotropically hardened remolded clays may have similar rigidity indices, though further testing is needed to confirm the finding.<br /><br />The results may help predict the behavior of offshore infrastructure and in situ tools that remold clay during installation.

Keywords

soft clays

thixotropic hardening

rigidity index

undrained shear strength

shear-wave velocity

small-strain shear modulus

Burgers model

plasticity

fall-cone tests

bender elements